Method for transmitting, storing and playing back a plurality of television signals

Abstract: A device for simultaneously transmitting a plurality of television signals comprises an adjusting circuit (5) for adjusting the bitrate for each television signal in dependence upon the complexity of this signal with respect to the joint complexity of the plurality of television signals. The device provides improvement of quality in Digital Video Broadcast systems. It has also been found to be possible to transmit extra signals through a given channel while maintaining the picture quality. Use of the device in optical recording provides the possibility of a longer playing time.

Description: FIELD OF THE INVENTION The invention relates to a method for transmitting a plurality of television signals through a transmission channel having a predetermined channel bitrate comprising: encoding each one of said plurality of television signals in a respective bit stream; combining the respective bit streams to a channel bit stream; and controlling the bitrate of each bit stream in proportion to the complexity of the corresponding television signal with respect to the joint complexity of the other television signals. The invention also relates to a method of storing a television signal and playing back said signal. BACKGROUND OF THE INVENTION A method defined in the opening paragraph is disclosed in NTC'77 Conference Record, Vol.3, 1977, New York, pages 41:4-1 to 41:4-6, H. Kaneko et al. "Digital Transmission of Broadcast Television with reduced Bit Rate." It allows a plurality of television signals to be simultaneously transmitted through one digital transmission channel, the channel capacity allocated to each signal corresponding with the signal complexity. The prior art device exploits the fact that upon transmission of a plurality of television signals through one transmission channel not all television signals are simultaneously complicated. For example, if two television signals A and B are transmitted and if, at a given moment, signal A comprises a scene which is not so complex and signal B comprises a very complex scene, signal B will be temporarily transmitted at a higher bitrate at the expense of the bitrate of signal A. This process of dynamic allocation of bitrates will hereinafter be referred to as "joint bitrate control". The method is suitable for use in Digital Video Broadcast (DVB) systems. Inter alia , the MPEG2 standard which is being developed is suitable for encoding each television signal. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to further improve the prior art method. To this end the method according to the invention, as set out in claims 1, 5 and 6, is characterized by the step of determining the complexity of each signal by encoding each picture of said signal and counting the number of bits for encoding said picture, the complexity being determined by said number of bits. The invention is based on the recognition that buffer occupancy is not an adequate representation of signal complexity because a plurality of encoded pictures are temporarily stored in each encoder buffer at any time. The invention overcomes this problem by determining the complexity in terms of the number of bits for encoding each picture. The complexity can be measured by a preanalysis circuit, for example in the form of an additional encoder. The complexity may also be computed from the number of bits and an average step size with which a previous picture of said television signal has been encoded. This possibility has the advantage that such a computation is already possible in MPEG encoders. It has been found that joint bitrate control reduces the risk of occurrence of a noticeably poor picture quality in each of the signals. The picture quality of the transmitted television signals thus roughly increases. However, joint bitrate control also provides the possibility of transmitting more television signals through the same transmission channel while maintaining the picture quality. It has been found that five television signals can be transmitted through a 20 Mbit/s channel at the same picture quality as four television signals having a constant bitrate of 5 Mbit/s. Further attractive possibilities of using joint bitrate control are obtained by dividing a television program into a plurality of parts of equal length and by simultaneously storing these parts as one channel bit stream on an optical disc by means of joint bitrate control. The television signal is reproduced by repeatedly reading the stored channel bit stream and by each time decoding a different part. Either a longer playing time or a higher picture quality is then obtained. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 shows a possible embodiment of a device for transmitting a plurality of television signals according to the invention. Fig. 2 shows a further embodiment of the device. Fig. 3 shows an alternative embodiment of an adjusting circuit shown in Fig. 2. Fig. 4 shows another embodiment of the device. Fig. 5 shows a device for storing a television signal according to the invention. Fig. 6 shows a device for playback of a television signal according to the invention. DESCRIPTION OF EMBODIMENTS Fig. 1 shows a device for transmitting a plurality of television signals according to the invention. A device for transmitting two television signals TV 1 and TV 2 is taken as an embodiment. Television signal TV 1 is applied to an encoder 1. This encoder comprises a Discrete Cosine Transformer 10 which transforms blocks of pixels into series of coefficients. The coefficients obtained are quantized at a given step size in a quantizer 11 and the quantized coefficients are subsequently encoded in a variable length encoding circuit 12. The code words obtained are applied to a buffer 13. The produced number of bits, which fluctuates from picture to picture, is applied to a bitrate control circuit 14 which, in response thereto, controls the step size of quantizer 11 in such a way that the average number of applied bits per picture is equal to a target value T 1 . Such a bitrate control circuit is known per se . In a corresponding manner, television signal TV 2 is processed in an encoder 2. This encoder also comprises a transformer 20, a quantizer 21, a variable length encoder 22, a buffer 23 and a bitrate control circuit 24. A target value T 2 is applied to the control circuit 24. The two bit streams are combined in a multiplexer 3 to one channel bit stream having a channel bitrate which corresponds to T = T 1 +T 2 bits/picture. The channel bit stream is applied to a transmission channel 4. In accordance with the invention, the device is provided with a joint bitrate adjusting circuit 5. This circuit determines a complexity value for both television signals (X 1 for the first television signal and X 2 for the second television signal) which is representative of the complexity of the current picture scenes, and distributes the available channel bitrate T among the two signals in proportion with their complexity. In a possible embodiment of the device the complexity values X 1 and X 2 are obtained by subjecting each applied television signal to a preanalysis. To this end the adjusting circuit 5 comprises a preanalysis circuit 51 and 52 for the two respective television signals. Such a preanalysis circuit comprises, for example a picture transformer, a quantizer having a fixed step size and a variable length encoder, as well as a counter for counting the number of bits thus obtained for each picture. In this embodiment said number of bits constitutes the complexity value X. The two complexity values X 1 and X 2 are added together in a summing device 53. Subsequently, the relative complexity of each television signal is determined in dividers 54 and 55. To this end the dividers compute: X 1 X 1 + X 2 and X 2 X 1 + X 2 respectively. Finally, the target values T 1 and T 2 are computed by means of multipliers 56 and 57 by multiplying the available channel bitrate T (expressed in bits/picture) by the relative complexity of the television signals. It now holds for the target values that: T 1 = X 1 X 1 + X 2 × T and T 2 = X 2 X 1 + X 2 × T If there are N television signals, it holds for the target value T n (n = 1 ... N) which is applied to the n th encoder: The device allocates a bitrate to each television signal in proportion with its complexity. A picture quality is then obtained which is substantially equal for all signals. However, it is alternatively possible to allocate a distinct picture quality to the television signals. This is achieved by influencing the relative complexity of the signals in a predetermined manner. In that case the dividers 54 and 55 are implemented, for example for computing: α 1 X 1 α 1 X 1 +α 2 X 2 and α 2 X 2 α 1 X 1 +α 2 X 2 respectively. in which α 1 and α 2 are constants. For signals of equal complexity (X 1 = X 2 ) an unequal bitrate is now obtained. Fig. 2 shows a further embodiment of the device. In this embodiment the encoders 1 and 2 are assumed to be MPEG encoders. As may be known (see for example ISO-IEC DIS 11172 "Coding of moving pictures and associated audio for digital storage media up to about 1.5 Mbit/s"), the television signal in the MPEG standard is transmitted in the form of Groups Of Pictures (GOPs). Each GOP comprises an intraframe coded picture (I picture), a number of predictively coded pictures (P pictures) and a number of bidirectionally predictive coded pictures (B pictures). For the sake of simplicity the customary prediction loop for forming a motion-compensated prediction picture in encoders 1 and 2 has not been shown in Fig. 2. In accordance with the invention the adjusting circuit 5 now comprises: (A) computing means 58 and 59 for computing the complexity X n of each of the N television signals to be transmitted. As an alternative to the preanalysis circuit shown in Fig. 1, the complexity value is now determined with reference to the number of bits S which is spent on coding the previous picture and a quantization parameter Q which is representative of the average step size with which this picture is quantized. Both values S and Q are applied to the adjusting circuit by each encoder. Chapter 10 of the afore-mentioned ISO document "MPEG2 Test Model 5" describes that the product of S and Q is a representative measure of the complexity of the television signal. By separately determining this product for each type of picture (I, P and B), three complexity values X 1 , Xp and X n are obtained for each television signal TV n (n = 1, 2): X In = S In × Q In , X Pn = S Pn × Q Pn and X Bn = S Bn × Q Bn Alternative formulas are possible. It is notably sensible to emphasize the influence of the quantization value on the complexity value as in, for example: X * = S * × Q * α (α > 1) or in X * = S * ×e Q * in which the index * refers to the type of picture (I, P, B). (B) a summing device 53 and dividers 54, 55 for determining, for each type of picture, the relative complexity of each signal in accordance with the equations: (C) computing means 50 for computing, for each type of picture, the total number of bits T for the subsequent image of the joint signals. Analogously to a stand-alone MPEG2 decoder (see chapter 10 of MPEG2 Test Model 5) the following computations can be performed: T I = R 1+ n P . X P X I . K P + n B . X B X I . K B , T P = R n P + n B . K P . X B K B . X P and T B = R n B + n P . K B . X P K P . X B in which n P and n B represent the number of P and B pictures still to be encoded in the GOP and K P and K B are specific system constants. In formula (1), X I , X P and X B represent the joint complexity of the television signals, as if their pictures jointly form a superpicture. This joint complexity value can be constituted by: or by: and To this end, the computing means 50 receive from each encoder its spent number of bits S and the average step size Q.In formula (1), R is the remaining number of bits which is allocated to the joint GOPs (superGOP). The initial value of R is the number of available bits/superGOP. For a stand-alone encoder this is denoted by G. In the device according to the invention, the initial value is N x G. R is reduced after encoding of each superpicture by the number of spent bits (S I , S P , S B , respectively). (D) multipliers 56 and 57 for distributing the target value T for the superpicture among the N encoders in proportion with the relative complexity of the corresponding television signal. In a formula this is expressed by: In the manner thus described, encoder 1 receives a target value T I1 , T P1 and T B1 for each type of picture (I, P or B). In a further autonomous manner, the control circuit 14 of the encoder will attempt to meet this target. In a corresponding manner, decoder 2 receives a target value T I2 , T P2 and T B2 . Thus it holds for each encoder that the applied target value is dependent on the relative complexity of the corresponding television signal with respect to the other television signals. Fig. 3 shows an alternative embodiment of adjusting circuit 5 for use in the device shown in Fig. 2. Identical reference numerals represent identical functions in this case. The adjusting circuit differs from the one shown in Fig. 2 in that the computing means ( cf . 50 in Fig. 2) are now split up into two identical computing means 501 and 502. They compute, for each television signal, the number of bits for the next picture as if the full channel bitrate were available for the signal. The afore-mentioned formula (1) applies to the computation, in which X I , X P and X B now represent the complexity value of the corresponding (instead of the joint) television signal. The number of bits thus computed is denoted by T' 1 and T' 2 in Fig. 3. It is this number which is multiplied (56, 57) by the relative complexity of the corresponding signal. The relative complexity is obtained in the same manner (summing device 53, dividers 54 and 55) as in Fig. 2. Fig. 4 shows another embodiment of the device according to the invention. This embodiment is distinguished from the previously described embodiments in that the quantizers 11 and 21 of a joint control circuit 60 receive an average step size which is equal for both encoders (if desired, the step size may vary from macroblock to macroblock within a picture, dependent on the activity). The control circuit receives a target value T which corresponds to the channel bitrate and controls the average quantization step size as a function of the number of bits which is jointly produced by the encoders. This number is supplied by an adder 61. The control circuit has the same structure as the known control circuit of a stand-alone encoder ( cf . 14 and 24 in Figs. 1 and 2) and does not need any further explanation. Since the step size for each encoder is equal in this embodiment ( i.e. Q 1 = Q 2 = Q), the complexity of each signal ( i.e. X n = S n .Q) is directly represented by the number of spent bits S n . A separate measurement of the complexity is therefore superfluous. As is apparent from S n = X n Q = X n X 1 + X 2 × X 1 + X 2 Q = X n X 1 + X 2 ×( S 1 + S 2 ) the bitrate for each signal is also proportional to its complexity in this embodiment. The picture quality is equal for both signals because both encoders quantize at the same step size. If a distinct picture quality is desired for each television signal, the step size generated by the control circuit 60 may be split up into two different step sizes which have a predetermined fixed ratio with respect to each other. For all previously described embodiments of the device it holds that the current bitrates of the N television signals can be transmitted to a receiving decoder in the form of appropriate parameters for each picture or for each GOP. The MPEG standard already provides this possibility. By using joint bitrate control, the picture quality of each television signal increases. This is a significant advantage for broadcast systems. Joint bitrate control also provides the possibility of transmitting extra signals. For example, instead of two stand-alone 5 Mbit/s television encoders, two centrally controlled encoders having a joint bitrate of 9 Mbit/s can be coupled to a 10 Mbit/s channel while maintaining the picture quality. Via the remaining 1 Mbit/s, for example an extra data signal can be transmitted. Theoretical considerations and practical experiments have further proved that a larger number of television programs can be transmitted while maintaining the picture quality, for example 5 programs instead of 4, or 27 programs instead of 20. In addition to broadcasting, joint bitrate control also provides interesting possibilities of application for recording. Fig. 5 shows a device for storing a television program on a storage medium. The device comprises means 100 for splitting the television program into two parts A and B of equal length. These means may be constituted by, for example two video recorders which play back the first and the second half, respectively, of the program. The two parts A and B are applied to a device 101. In the manner described hereinbefore, device 101 forms one channel bit stream A+B of the two parts, which channel bit stream is stored on an optical disc 102. Fig. 6 shows the corresponding device for playback of the optical disc. The channel bit stream A+B read from disc 102 is applied to a selector 103 which first selects bit stream A and applies it to an MPEG2 decoder 104. The decoded signal is displayed on picture monitor 105. Under the control of a control circuit 106 the same channel bit stream is subsequently read again after finishing of part A. Now, however, bit stream B is selected and decoded. During reset of the read head (approximately 0.8 sec in practice) the last picture of part A may be displayed as a still picture. Should this be troublesome, a buffer may be used for bridging this period. This application of joint bitrate control in recording requires a disc rotating at a (substantially) double speed and a more complex disc control. This is, however, offset by a better picture quality or a longer playing time.

Claims: A method of transmitting a plurality of television signals through a transmission channel having a predetermined channel bitrate (T), comprising: encoding (1,2) each one of said plurality of television signals in a respective bit stream; combining (3) the respective bit streams to a channel bit stream; controlling (14,24) the bitrate of each bit stream in proportion to the complexity (X 1 ,X 2 ) of the corresponding television signal with respect to the joint complexity (ΣX i ) of the other television signals; characterized by the step of determining the complexity of each signal by encoding each picture of said signal and counting the number of bits for encoding said picture, the complexity being determined by said number of bits. A method as claimed in claim 1, wherein said step of determining the complexity includes applying the picture to be coded to a preanalysis circuit (51,52) comprising a quantizer having a fixed step size. A method as claimed in claim 1, wherein said step of determining the complexity includes counting the number of bits spent on coding a previous picture, the complexity being further determined by the average step size with which said previous picture has been quantized. A method as claimed in any one of the preceeding claims, wherein said encoding is MPEG encoding including autonomous coding of I-type pictures, predictive coding of P-type pictures, and bidirectional predictive coding of B-type pictures, the picture used for determining the complexity being a picture of the same type as the picture to be coded. A method of storing a television signal on a storage medium, comprising the steps of: splitting (100) said television signal into a plurality of signal parts of equal length, encoding (101) each part in a respective bit stream; controlling (14,24) the bitrate of each bit stream in dependance upon the complexity of the corresponding signal part with respect to the joint complexity of the plurality of signal parts; combining (3) the respective bit streams into a combined bit stream; and storing said combined bit stream on the storage medium. A method of playing back a television signal stored on a storage medium using the method as claimed in claim 5, comprising the steps of: repeatedly reading (106) a stored channel bit stream which comprises a plurality of simultaneously occurring signals, the bit rate of each signal being dependent on the complexity of said signal with respect to the joint complexity of the plurality of signals; selecting and decoding (103, 104) a predetermined one of the simultaneously occurring signals from the read channel bit stream.

Inventors: KEESMAN GERRIT JOHAN, KEESMAN, GERRIT JOHAN, KEESMAN, GERRIT JOHAN, C/O INT. OCTROOIBUREAU B.V.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1994
Main classification: H04N7
IPC codes: H04N7, H04N5